Method and apparatus for obtaining mass average samples from a liquid stream

ABSTRACT

An improved technique of extracting small samples from a liquid stream, such as a storm sewer, wherein each sample contains a representative portion from all levels of the stream. The relative sample proportions of liquid mass extracted from each elevation of the stream are directly related to the width of the stream at that elevation and the average velocity of the stream at that elevation. An apparatus is disclosed for carrying out this technique wherein a substantially continuous flow pump traverses across the liquid stream in a vertical direction with a rate of speed alongits path that is a function of a product of a reciprocal of the velocity and width functions along the height of the stream.

United States Patent 091 Schwind [54] METHOD AND APPARATUS FOR OBTAINING MASS AVERAGE SAMPLES FROM A LIQUID STREAM [75] Inventor: Richard G. Schwind, Lahonda,

' Calif.

[73] Assignee: Nielsen Engineering 8: Research,

Inc.

[22] Filed: Nov. 22, 1971 21 Appl. No.: 200,750

[ 51 Apr. 3, 1973 2,872,818 2/1959 Johnson ..73/42l B Primary Examiner S. Clement Swisher Attorney-Karl A. Limbach et al.

[57] ABSTRACT An improved technique of extracting small samples from a liquid stream, such as a storm sewer, wherein each sample contains a representative portion from all levels of the stream. The relative sample proportions of liquid mass extracted from each elevation of the stream are directly related to the width of the stream at that elevation and the average velocity of the stream at that elevation. An apparatus is disclosed for carrying out this technique wherein a substantially continuous flow pump traverses across the liquid stream in a vertical direction with a rate of speed alongits path that is a function of a product of a reciprocal of the velocity and width functions along the height of the stream.

8 Claims, 7 Drawing Figures PATENTEDAPRS ms 3.724.276

SHEET 1 OF 3 WATER HEIGHT IN SEWER PIPE 0 0 vzwurv 0F SAMPLEK m I A DIRECT/0N "h' FIE--2-- 1 15-54- INVENTOR.

BYE/CHAR!) SCHW/ND gmimum ATTOKNEYS PATENTEDAPRS ms 3.724.276 SHEET 2. BF 3 F 1 F YRICHARD FEE/fi m!) zaJu/blmifim ATTOKNEYS METHOD AND APPARATUS FOR OBTAINING MASS AVERAGE SAMPLES FROM A LIQUID STREAM BACKGROUND OF THE INVENTION This invention relates generally to liquid stream sampling systems, particularly the sampling of a sanitary or a storm sewer in order to determine the pollutant content thereof.

Knowledge of the quality and quantity of flow in sewers'is becoming increasingly important because of the desire to provide better treatment to the pollutant load that is carried in sewers. In order to provide the most efficient and economical treatment, it is necessary to know what the pollutants are and how the pollutant load varies with time.

In sanitary and combined sewers, it is becoming more common to require industrial and commercial firms to monitor their effluent dumped into the municipal sewer systems in order that the firms can be billed on the basis of their pollutant load injected into the sewers to help support the cost of sewer system operation. In order for firms to monitor and establish their pollutant loads for billing purposes, it is necessary to have representative samples of the flow in the sewer.

Further, in waste water treatment plants, it is highly desirable to obtain samples of the incoming sewer water over the depth of the flow to insure that any local concentrations of pollutants are sampled and that accurate loads of all pollutant levels are measured.

Floatable constituents of the sewer water, such as oil, grease, and wood materials, travel along the surface of the sewer water. Throughout the depth of the water will be suspended solids of dissolved materials such as chemicals, pesticides, nitrogen and phosphate nutrients, dirt, rags, and paper. Near and at the bottom of the sewer water will be the heavier material which may be in suspension or may be rolling or bumping along the bottom of the sewer. If the sewer water flow is very turbulent and well mixed, the concentration of each of the various pollutant constituents carried thereby will tend to be uniform as a function of the stream depth. However, the degree of mixing of the various pollutants probably changes in a given sewer with the flow velocity. The degree of mixing also varies between sewers and is dependent upon the sewers physical characteristics which promote turbulence such as bends, enlargements, and spillways.

Existing sampling techniques and apparatus primarily extract a sample ,at only one point in a cross-section of a storm sewer water flow. This has the disadvantage that the sample does not contain a representative portion of all the pollutants in the water flow unless the flow happens to be homogeneous and well mixed. Therefore, it is a primary object of the present invention to obtain a sample from a liquid flow that contains pollutant constituents in the same proportion as carried by the total mass of sewer water.

It is further an object of the present invention to provide an improved sampling pump for immersion in the liquid flow which does not accumulate debris or easily become plugged.

SUMMARY OF THE INVENTION According to the technique of the present invention, varying sample quantities are obtained at different elevations within the liquid stream at a single location along the length of the stream. A sample is obtained by extracting a portion thereof at each level within the liquid stream according to a distribution that is directly proportional to a product of a horizontally average liquid velocityand the width of the stream as functions of elevation. This assures that the proportion of each constituent obtained in the sample is substantially the same as the proportion of that constituent in a given large mass of the liquid stream, even if the various constituents are not evenly distributed along the depth of the stream. A sample is preferably extracted continuously along a vertical line extending between the bot! tom and the top of the stream.

In any liquid stream, the velocity of its travel is significantly less near its bottom adjacent a solid containing boundary than at a midpoint of the stream. There fore, the volume or mass of liquid that flows along the stream in an increment of height per unit time is significantly less near the bottom of the stream than at some midpoint elevation of the stream. Furthermore, variations in width of the stream at different elevations cause the mass flow per increment of height per unit time to vary at different elevations of the stream. The techniques of the present invention take into account the effects of these velocity and width variations as well as any density gradients, if significant enough to be accountable. A sample portion obtained from one incremental height of a liquid stream is substantially the same as the ratio that the mass flow per unit time of that incremental height of the stream bears to the total mass flow per unit time of the entire stream.

The sample collecting technique according to the present invention can be directed toward either the collection of separate samples or the accumulation of a single sample. Several samples may be taken in succession and each of these samples may either be stored in separate containers as a means of monitoring the changing constituent composition of the liquid flow, or the individual samples may be deposited in a single container for determining an average constituent composition of the liquid flow over a longer length of time. In using this latter technique, existing proportionality methods may also be employed wherein the individual samples are taken at intervals between which equal volumes or masses of liquid pass along the stream. When the velocity or height of the stream flow increases, the rate of taking samples increases.

One convenient way of implementing the sampling technique of the present invention is to draw a pump inlet at a uniform speed across a liquid stream from its bottom to its top in substantially a vertical path. The pump suction is not constant, however, but rather it is made to vary as a function of the position of the pump inlet along the height of the stream. The rate of sample volume flow varies along the height of the stream directly I as a product of a vertical stream velocity profile and width functions vary. At elevations of the liquid stream having a low velocity of flow or. a narrow width, the pump suction at its inlet is at a low level. An apparatus for carrying out this method according to the present invention is described in detail in a copending application of Jimmie Crumal, entitled Apparatus for Extracting a Liquid Sample Over a Range of Depths of a Liquid Stream."

Another way of implementing the technique of the present invention is to maintain constant the rate at which a sample is extracted from a liquid stream and vary instead the speed at which a pump inlet is drawn vertically across the liquid stream. The speed of the pump inlet as a function of vertical position across the stream is inversely proportional to a velocity profile and width function of the stream along the vertical line. That is, at low flow velocity or narrow width portions of the stream, the pump opening is made to travel rapidly while at high velocity or wide portions of the stream the pump opening is made to slow down.

This second method of implementing the present invention is described hereinafter in detail and includes the use of an improved pump that is caused to traverse vertically across a liquid stream. The sampler pump is immersed in the liquid flow only during the time that a sample is being taken, thus reducing the chance of blocking the stream by accumulation of debris. Existing samplers which have a tube depending downward into the liquid stream on a permanent basis tend to promote blockage of the sewer. The sampler described hereinafter also traverses the entire depth of the liquid and has provisions for accommodating varying depths. This provides a more accurate sample than can be obtained by existing samplers that extract a sample at only a single elevation off the bottom of the stream regardless of the streams level. Additionally, the sampler unit preferably can accept large particles near the bottom of a liquid flow without plugging. A sampling technique, and apparatus having these features is particularly useful in the application of monitoring a storm sewer.

Additional objects and advantages of the present invention will become apparent from the following description when taken in conjunction with the drawings which describes a preferred embodiment of the present invention that is particularly suited for monitoring flows in sewers.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a round storm sewer;

FIG. 2 shows velocity and width profiles of the storm sewer of a type shown in FIG. 1;

FIG. 3 graphically shows a desirable variation of velocity of the sampler across a liquid flow in the storm sewer of FIG. 1;

FIG. 4 is a side view of the principal components of a preferred sewer sampler apparatus;

FIG. 5 shows across-sectional view of the pump unit of FIG. 4;

FIG. 6 is an enlarged view of a portion of the pump of FIG. 5 taken across section 6-6; and

FIG. 7 shows in block diagram form the principal elements of a control system for the sampler of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 schematically shows a circular storm sewer pipe 11 having various quantities identified as an aid in analyzing sewer water flow through the sewer. A sample of sewer water is extracted along a substantially vertical line 12 that passes from the top to the bottom of the sewer pipe 1 l and through its center. The surface of the fluid, indicated at 65, can be at any location, or the conduit can be full. It will be understood that the techniques of the present invention are not limited to extracting samples from a storm sewer pipe but are also of advantage in extracting samples from nonhomogeneous liquid flows of other types, including flows in stream beds. However, the techniques of the present invention have particular advantage to sampling of sewer water flow.

Consider an incremental volume 13 of liquid flowing in the pipe 11 that has a height N1, and a distance h from the bottom of the storm sewer 11. The width of the incremental volume 13 is indicated by b, and its velocity along the length of the storm sewer is indicated by V The mass of liquid which passes by a given point of the sewer pipe 11 in a given time in the incremental volume 13 is directly related to the width b and the velocity V, of the incremental volume. A certain small percentage of this mass of sewer water is removed as part of a sample of the total flow in the sewer pipe 1 l.

The small percentage of a mass of sewer water is removed from every other incremental volume within the sewer water flow. Consider a second increment 14 of the total flow. Its width 12 is less than that (b,) of the first increment. Also, the flow velocity V is less than the flow velocity V of the first increment 13 since the second increment 14 is more restrained by friction with the bottom of the sewer pipe 11. The same small percentage as was taken from the mass of sewer water flowing in the first increment 13 is taken from the mass of sewer water flowing in the second increment 14 as a second sample. These two samples are then mixed together to form a composite sample with constituent proportions being substantially the same as the constituent proportions of the combined flows of the increments 13 and 14.

The same procedure can be followed for the remaining incremental volumes of Ah high across the stream of sewer water within the pipe 1 1. This is, of course, impractical but the beneficial results of such a technique can be realized by continuously extracting sample liquid by scanning a pump inlet through the stream along the substantially vertical path 12. The technique of the present invention compensates for a varying velocity of liquid flowing at different levels in the sewer pipe 11 as well as compensating for variations in the width of the sewer pipe.

Referring to FIG. 2, characteristic curves of the sewer pipe 11 and a liquid flow therein are shown. Regardless of the height of liquid within the sewer pipe 1 l, a velocity profile of the sewer pipe 11 takes on a characteristic form such as that indicated by the curve V of FIG. 2. The curve V shows the relative velocities of fluid flowing at different elevations within the sewer pipe 11 along the vertical sample line 12. The various points on this curve are average velocities across the horizontal dimension. Such a curve can be constructed for a given location along a sewer pipe by empirical observation coupled with theoretical calculation.

The width of the sewer pipe 11 as a function of its height h is shown by the dotted curve b. Of course, other shaped sewer pipes will have a different characteristic curve. For instance, a square sewer pipe would have a step function type of curve b wherein the width is uniform from the bottom to the top.

A product of the velocity profile curve V and the width curve b, when multiplied by the average density p of the sewer water, gives a characteristic mass flow distribution stream passing through the pipe 11 per unit time as a function of height h within the stream. In order to obtain a mass average sample according to the techniques of the present invention, the mass of the sample extracted as a function of the height h of the sewer pipe 11 substantially conforms to the total mass flow w. That is, the relative proportions of a sample that is withdrawn from the liquid flow within the sewer pipe 11 at various elevations therein along the vertical line 12 follow the shape of the mass flow curve w of FIG. 2. At a height h where the magnitude of w is low, such as near the bottom of the pipe 11, the portion of a sample extracted at that level is low. However, where the magnitude of the curve w of FIG. 2 is high, such as near the middle of the pipe 11, the proportion of liquid sample withdrawn at that height h is high.

A preferred technique for extracting a sample according to these requirements is to immerse a liquid pump inlet within the liquid flowing in the pipe 11 and moving it along the vertical line 12 while the pump operates to remove a uniform volume of sample per unit time. In order to obtain the desired variation in the amount of sample obtained at various heights across the pipe, as discussed above, the pump inlet is traversed vertically along the line 12 at a varying rate of speed. The velocity function of the pump inlet is proportional to the inverse of the total mass flow curve w of FIG. 2. This velocity function is shown in FIG. 3 wherein a given water height h, within the sewer pipe 11 is assumed to be less than the maximum height of the sewer. At positions along the vertical line 12, wherein the rate of sewer water mass flow is low, the pump inlet travels at a high rate of speed so that a low amount of sample is extracted therefrom. Conversely at positions across the stream where the sewer water mass flow rate w is high, the pump input slows down in its travel across the stream to obtain a larger proportion of the sample from those positions across the stream. Of course, the rate of speed of the pump inlet travel must reach zero at the bottom of the pipe and at the top surface of the liquid flowing therein in order to reverse direction.

Referring to FIG. 4, an apparatus is shown for perlating frame 29 for rotatably driving a pinion gear 43 that mates with the rack 39 to give motion to the translating frame 29 and thus to the pump head 31 within the sewer waters.

Power is applied to the pump by means of a second electric motor 45 that drives the elongated tubular member 33 by means of a chain or some other rotatable connection 49. At the end of the tube 33 is the pump unit 31 which includes a partial admission rotor 51 that is attached to the rotating elongated tube 33. Rotation of the partial admission rotor 51 forces fluid from the sewer through the stator 71 into a second elongated tube 53 that is positioned concentrically within the rotating tube 33. The elongated tube 53 is fixed to thetranslating frame 29 while the outer rotating tube 33 is connected to the translating frame 29 through appropriate bearings that allow rotation but no longitudinal motion with respect to the translating frame 29. Liquid pumped up through the non-rotational tube 53 is guided to a three-way solenoid valve 55 by a flexible tube 57. The solenoid valve 55 is of a type that can direct liquid flowing in the flexible tube 57 either to an outlet 59, for discharge back into the sewer 15, or into an accessory package 61 by means of a liquid tube 63. The accessory package 61 contains the power supply and control elements for the sampler, and also contains a rack having a plurality of sample containers and a mechanism for indexing the rack in a manner that sample sewer water passing through the tube 63 will go into a different container each time a new sample is taken.

forming the sample extraction by' varying the rate of 1 speed at which a pump inlet is traversed vertically across a sewer water stream. The sampler is shown in a typical installation wherein a sewer pipe 15 includes a manhole 17' communicating with the sewer and a ground level 19. A manhole cover 21 provides access to the sewer and to the sampler that is installed within the manhole 17. A mounting pad 23 of the sampler unit is rigidly mounted on the side of the manhole through a sewer mounting pad 25. A box-like frame 27 of the sampler unit is rigidly attached to the sampler mounting pad 23.

A translating frame 29 slides up and down within the sampler frame 27 as a means of translating a pump 31 at the end of elongated tubular member 33 up and down within the sewer water 35. The translating frame 29 slides up and down on a guide bar 37 that is rigidly attached to a side of the sampler frame 27. A rack 39 is also positioned vertically and fixed to the sampler frame 27. An electric motor 4lis attached to the trans- Between the samples, the sampler of FIG. 4 is at rest with the pump unit 31 drawn up above the surface of the sewer water 35 and immediately adjacent the underside of the fixed sampler frame 27. Since nothing depends downward into the liquid sewer flow 35 except when a sample is being taken, the sampler provides very little undesirable obstruction to the flow of the sewer waters. When a sample is to be taken, the pump drive motor 45 is energized which rotates, with respect to the translating frame 29 of the pump, the outer elongated tube 33 and thus the partial admission rotor 51 of the pump. The solenoid 55 is initially in a position which causes liquid to be discharged back through the pipe 59 into the sewer 15.

The translating drive motor 41 is then energized which causes the pump to travel downward until it strikes a surface 65 of the sewer water 35. The height of the surface 65 is sensed at this point and recorded in the control circuits within the accessory package 61. The pump 31 continues its descending course down through the sewer water 35 until it reaches its bottom 67 at which time the drive motor 41 reverses and pulls the pump 31 upward. The pump 31 will only rise to the surface 65 of the sewer water 35 until it is again reversed in direction to head downward. The pump 31 so reciprocates through the sewer water flow for several cycles until a well mixed sample flow is established through the tube 53' from the pump 31. When this occurs, the control circuit within the accessory package 61 causes the solenoid valve 55 to switch to a position so that the flow in 'the tube 57 from the pump flows through the tube 63 and into a bottle for collecting a sample. This flow continues for one complete translational cycle of the pump head 31; that is, a sample is collected while the pump head 31 travels downward from the surface 65 of the sewer water 35 to the bottom 67 and back again to the surface 65. After this cycle has occurred. the pump head 31 is again withdrawn from the sewer water 35 and the solenoid 55 is returned to its position for discharge of the sample drawn by the pump 31 through the hose 59 back into the sewer water 35. The fluid in the tube 53 and the tube 57 will then drain out through the pump head 31. The sample bottle rack within the accessory package 61 is then indexed and an empty bottle is placed in a position to receive a new sample through the tube 63 when the next sample is taken.

The pump 31 in its preferred construction is shown in more detail in FIGS. and 6. The rotating tube 33, driven by the motor 45 of FIG. 4, imparts rotary motion to the partial admission rotor 51 which is generally solid except for one or more openings with rotor blades 69 firmly attached thereto. The rotor 51 is driven in a direction to cause the rotor blades 69 to force liquid radially inward. A stator 71 includes a plurality of stator blades 73. The rotor 51 imparts velocity energy to liquid and this energy is partially converted into a static pressure increase by the stator blades 73 as the liquid flows radially inward and up through the interior of the stationary tube 53. The rotating tube 33 and the stationary tube 53 are held concentrically with respect to one another and held spaced apart by appropriate means such as a bearing 75 shown in FIG. 5.

The diameters of the stator 71 and of the rotor 51 of the pump 31 shown in FIGS. 5 and 6 are made to be large enough to keep the rotational speed low to keep vibration to a minimum. The height of the rotor blades 69 and associated stator blades 73 are made large enough to handle prevalent particles of sewer water in order to prevent plugging of the pump. However, with such a large diameter rotor and stator and with the high rotor and stator blades, an excessive amount of sample liquid may be withdrawn if the rotor 51-includes the blades 69 completely around its perimeter. Therefore, the portion of the perimeter of the rotor 51 with an opening for fluid to flow therethrough is limited as a means of limiting the amount of sample that is withdrawn. Placing the rotor blades on the outside of the pump stator has an advantage of being able to knock away very large objects in the sewer water, such as boards or large rocks, and thus prevent such objects from interfering with the pump operation.

The translating frame 29 and the pump assembly 31 as shown in FIG. 4 are not translated up and down at a constant rate of speed but rather are translated with a velocity that is a function of the height within the sewer 15. The velocity function is inversely proportional to a product of the velocity profile (V of FIG. 2) of the sewer and its width (b of FIG. 2) as a function of the height (h of FIG. 2) of the sewer. The velocity of travel of the sampler pump is made to conform as closely as possible with the inverse of the mass flow function w, as discussed hereinabove with respect to FIG. 3. This varying velocity is accomplished by varying the speed of the electric motor 41 of the FIG. 4. The motor 41 is preferably of a direct current type so that the voltage applied to its field windings directly controls the speed of the pinion gear 43, and thus the speed of travel of the pump 31 vertically through the sewer water 35. There are a number of known function generators that can be inserted electrically in series with the power supply to the motor 41 for controlling the velocity of the pump 31 as a function of its position along the height of the sewer 15.

FIG. 7 schematically illustrates a control system for operating a sampler apparatus of FIGS. 4-6 in the manner described above. A direct current voltage source (not shown) is connected at terminals 81 and 83. A push-button switch S1 is connected with the terminal 81 and is of a type to make a momentary contact so long as pressure is applied thereto. Another switch S2 is parallel with the switch S1. When the switch S1 is depressed, the sampler apparatus is energized and begins its sequence. The switch S1 locks the switch S2 in a conductive state. At the end of the sequence the switch S2 is moved to disconnect the sampler from its power source.

When the sampler is energized, the motor 45 causes the pump rotor to be given motion. The translating motor 41 is also energized which causes the pump head 31 to descend. An appropriate surface sensor 85 is provided which generates a signal when the pump head strikes a surface 65 of the sewer water stream 35. A function generator 87 takes this signal and evaluates the depth of the liquid stream. The height of the pump head 51 when it first strikes the liquid surface 65 is also sent to the function generator 87 by a potentiometer R1. The potentiometer R1 is mechanically driven by the translating motor 41 and thus emits a signal which is proportional to the height of the pump head 57 within the sewer 15. The surface sensor 85 may utilize a pair of terminals 89 and 91 which are attached to the bottom of the pump head 31 so that an electrically conductive path is formed between them when the pump head 31 first reaches the liquid stream surface 65.

The function generator 87 sends a varying voltage to the translating motor 41 through contacts of a relay Re- 1. The voltage varies as a function of the height of the pump head 31 in a manner discussed above with respect to FIG. 3. Since the height of the surface 65 of the liquid stream 35 has been noted by the function generator87, a signal may be transmitted through a line 93 therefrom to the relay coil each time the potentiometer R1 sends a signal to the function generator 87 that indicates the pump head 31 is at the liquid stream surface elevation. This signal will cause the relay Re-l to change its state which results in reversing the polari ty on the dc. translating motor 41. By this method the pump head 31 is caused to translate back and forth across the stream 35 a specific number of times after which the function generator 87 sends a signal to the solenoid valve 55 through a line 95. The solenoid 55 is switched to divert sample fluid into a container. After one complete translational cycle of the pump head 31, the pump head 31 is removed from the stream and returned to its rest position at which time the switch S2 is opened to remove all power from the sampler until the switch S1 is again momentarily depressed. The switch S1 may be replaced with a timer for unmanned sampler operation.

The present invention has V been described with respect to a specific sampling method and apparatus, but it will be understood that the scope of the invention is limited only by the scope of the appended claims.

What is claimed is:

determining a stream flow relative velocity functionalong a substantially vertical line across said con"- tainer,

substantially vertical line across said. container,,

and

removing a sample quantity of liquid along said substantially vertical line, portions of said sample being collected all along said line according to a relative distribution that is a function of a product of the relative velocity and width functions of said container across said line.

2. The method as defined by claim 1 wherein the step of removing a sample includes drawing a liquid pump inlet along said substantially vertical line with a velocity that varies along said line inversely as a function of a product of said relative velocity and width functions, the pumping rate of said pump being maintained substantially constant. y

3. The method as defined by claim 1 wherein the step of removing a sample includes drawing a liquid pump inlet at a uniform speed along said substantially vertical line, the pumping rate caused to vary along said line directly as a function of a product of said relative velocity and width functions.

4. A method of extracting a liquid sample from a stream flowing in a fixed conduit having given flow velocity and width variations as a function of vertical position across said conduit, comprising the steps of:

translating-a pump inlet with continuous motion vertically across said conduit, said pump operating to remove a substantially constant mass of liquid as a function of time,

controlling the speed of said pump inlet translation as a function of position across said conduit ac cording to a function that varies inversely with a product of said given conduit flow velocity and width functions, and

collecting into a container the liquid sample discharged by said pump. 5. The method as defined by claim 4 wherein the g sample is collected into the container during at least determining a container width function along said" one full traverse of said pump inlet from the stream surface to the bottom of the conduit.

6 The method as defined by claim 5 wherein said fixed conduit is a sewer and said stream is a sewer flow.

7. Apparatus for sampling a sewer water in a sewer of fixed dimensions, comprising:

a liquid pump, a sample flow inlet opening to said pump, a discharge opening communicating with said pump, means for vertically translating said pump inlet opening continuously across at least a portion of said sewer, and means for varying the speed of the pump inlet translation across the sewer as a function of the pump inlet's vertical position in the sewer according to a function that is inversely proportional to the total mass flow of sewer water as a function of its height. 8. A sampling apparatus according to claim 7 wherein said pump comprises:

a rotationally fixed circularly shaped stator having a lural' of enerall radiall extendin stat lades iixedly attachgd therefo and adja cent iii outer circumferential edge of the stator,

a circularly shaped rotor surrounding said stator and having a plurality of blades positioned in an opening along a circumferential surface of. the rotor to force liquid radially inward through said stator blades, said rotor opening being said pump inlet,

means for conducting liquid from within the stator blades to said discharge opening, and

means for giving rotary motion to said rotor with a uniform angular velocity. 

1. A method of extracting a liquid sample across a vertical line of a liquid stream flowing through a container such as a pipe, comprising the steps of: determining a stream flow relative velocity function along a substantially vertical line across said container, determining a container width function along said substantially vertical line across said container, and removing a sample quantity of liquid along said substantially vertical line, portions of said sample being collected all along said line according to a relative distribution that is a function of a product of the relative velocity and width functions of said container across said line.
 2. The method as defined by claim 1 wherein the step of removing a sample includes drawing a liquid pump inlet along said substantially vertical line with a velocity that varies along said line inversely as a function of a product of said relative velocity and width functions, the pumping rate of said pump being maintained substantially constant.
 3. The method as defined by claim 1 wherein the step of removing a sample includes drawing a liquid pump inlet at a uniform speed along said substantially vertical line, the pumping rate caused to vary along said line directly as a function of a product of said relative velocity and width functions.
 4. A method of extracting a liquid sample from a stream flowing in a fixed conduit having given flow velocity and width variations as a function of vertical position across said conduit, comprising the steps of: translating a pump inlet with continuous motion vertically across said conduit, said pump operating to remove a substantially constant mass of liquid as a function of time, controlling the speed of said pump inlet translation as a function of position across said conduit according to a function that varies inversely with a product of said given conduit flow velocity and width functions, and collecting into a container the liquid sample discharged by said pump.
 5. The method as defined by claim 4 wherein the sample is collected into the container during at least one full traverse of said pump inlet from the stream surface to the bottom of the conduit.
 6. The method as defined by claim 5 wherein said fixed conduit is a sewer and said stream is a sewer flow.
 7. Apparatus for sampling a sewer water in a sewer of fixed dimensions, comprising: a liquid pump, a sample flow inlet opening to said pump, a discharge opening communicating with said pump, means for vertically translating said pump inlet opening continuously across at least a portion of said sewer, and means for varying the speed of the pump inlet translation across the sewer as a function of the pump inlet''s vertical position in the sewer according to a function that is inversely proportional to the total mass flow of sewer water as a function of its height.
 8. A sampling apparatus according to claim 7 wherein said pump comprises: a rotationally fixed circularly shaped stator having a plurality of generally radially extending stator blades fixedly attached thereto and adjacent an outer circumferential edge of the stator, a circularly shaped rotor surrounding said stator and having a plurality of blades positioned in an opening along a circumferential surface of the rotor to force liquid radially inward through said stator blades, said rotor opening being said pump inlet, means for conducting liquid from within the stator blades to said discharge opening, and means for giving rotary motion to said rotor with a uniform angular velocity. 